1. Field of the Invention
The present invention and the invention described in copending application Ser. No. 330,616 filed on Dec. 14, 1981, relate to the field of electronic, switching-mode power supplies and, in particular, to regulator circuits for electronic switching-mode power supplies employing blocking oscillators.
2. Prior Art
The use of switching elements in electronic power supplies is well known in the present state of the art. The advantages of such power supplies include higher efficiency, lower weight and smaller size in comparison to analog power supplies. At some power levels, switching-mode power supplies are even less costly than their analog counterparts.
The size and weight advantages of switching-mode power supplies are achieved by operating their transformers and other magnetic components at high frequencies. In a conventional power supply, the transformer is operated directly from the main power source and, accordingly, is operated at the frequency of the power source.
The size advantages of commercial switching-mode power supplies results from the operation of the power transformers at a frequency well above that of standard power line frequencies. In fact, it is usually well into the high audio frequency or ultrasonic frequency range. Dramatic miniaturization is thus achieved, albeit at the expense of somewhat greater circuit complexity.
For the same power levels, a conventional transformer will vary in size approximately inversely with frequency. As frequencies become higher and higher, cores having suitable core loss characteristics cause the relationship to become less favorable since the so-called "low-loss" materials may have low maximum flux density capabilities. Thus, the core size itself will be larger than would be predicted if a change in core material was not required. Nonetheless, transformers having extraordinarily high volt-amp ratings per unit volume, are made possible by operation at the high frequencies possible with switching-mode circuitry.
Because the switching-mode power supply is lightweight and has such superior compactness, it has become more and more the circuit of choice for small, semi-portable equipment. In fact, the use of switching-mode supplies is now being seen in applications which were once thought to be the exclusive domain of analog supplies such as in small digital computers, in particular those intended for small business applications, where compactness is considered an important attribute for ease of installation in an office environment.
The conventional approach to design of switching-mode power supplies has been to employ a magnetically-coupled multivibrator which uses a pair of high-efficiency, solid-state switches, each alternately switching one-half of a center-tapped transformer primary to cause a square-wave having peak voltage equal to twice the center-tap voltage to appear across the entire primary. On alternate half-cycles, the primary current flows first in one side of the primary through the switch which is on, then through the other side of the primary and its associated switch, each for one-half of the period of the supplier basis operating frequency.
The square-wave is then stepped up or down by appropriate secondary windings and, usually, rectified and filtered for supply to a direct-current load.
Regulation of the output may be achieved, if desired by either a dissipative regulator or a switching-mode regulator. The series-pass transistor for the regulator may be located in either the primary or the secondary side of the transformer. For regulation of an output voltage which is lower than the input voltage, the most efficient choice is usually to locate the pass transistor in the primary side and to close the feedback loop aroung the transformer and rectifier/filter combination.
Although the magnetically-coupled multi-vibrator is a straightforward and relatively power efficient circuit which is unaffected by wide ranges of load variation, it is also relatively expensive due to the need for the three power semiconductors, two of which are required for the functions of chopping and one of which is required for the function of regulating the input direct current. Indeed, for certain applications, line-voltage and power levels would require transistor specifications which are beyond the state of the art. For the highest possible voltage, at the minimum, a "bridge" primary switch is required, at approximately twice the complexity of the standard design. In addition, the regulator control circuitry for the standard regulated DC-DC converter is at least as complicated and expensive as its analog counterpart.
The search for more cost-effective ways to achieve a regulated switching-mode power supply has led to the adoption in recent years of the blocking oscillator and its variants as the basic power converter design. Although somewhat touchy in terms of start-up and wide load-range operation, the blocking oscillator is a highly efficient circuit both in terms of its power processing efficiency and its parts cost. Instead of a pair of switching transistors and a series-pass transistor, the blocking oscillator-based power supply requires but a single switching transistor which can be made to perform the functions of both chopping the unregulated direct current supplied to the input, and regulating the voltage produced at the output.
In addition to the reduction in parts count, the blocking oscillator-based power supply can be rendered in a design which does not require the switching transistors to see twice the input voltage, as does the standard DC-DC converter. Instead the power switch sees a theoretical maximum voltage of significantly less than twice the input voltage, depending upon the duty cycle which is chosen for its operation. Thus, operation of the supply directly from a 220 volt rectified main power source is possible, even using currently available semi-conductor devices.
The desire for higher and higher efficiencies has led to the development of subtle circuit refinements, which allow the blocking oscillator to operate reliably over wider ranges of input voltage and load voltage. Nonetheless, there are areas within the circuit which are wasteful of power and can still be improved by further refinement, namely in the areas of reducing excessive base drive and providing a reliable current-limit point for controlling the pass-transistor characteristics during overload fault and during turn-on, and power-official load like control so that the main power switch always remains within the safe-operating area of its specifications.
In the past, safe operating area operation has been assured by providing a capacitor bypass, or "snubber", for current flow during the fall time of the power switch, so that the switch itself can be turned off sufficiently to avoid instantaneous peaks of high-current and high-voltage during turn-off, a potentially catastrophic condition. However, the current which flows during the fall time can be advantageously used to improve operation of the auxiliary current-limit circuitry. Past current limiter circuitry has been employed to prevent over-current conditions during turn-on and during load faults, but operation of simple versions of these circuits has been unpredictable due to the large variations in voltage sensing components which are employed thus causing unreliable predictions of the current limit point from unit to unit.